Epoxy resins and their compositions having inorganic fillers blended therein are widely used as molding material, powder coating material, electrical insulating material and the like because of their moldability, bond strength, electrical properties, mechanical properties, and humidity resistance. In recent years, they have been used as encapsulating materials for semiconductor chips.
However, prior art epoxy resin compositions tend to crack upon curing, forming molded and coated articles having a less desirable appearance. Particularly, when they are used for the encapsulation of semiconductor devices, most of them are likely to produce defects in semiconductor elements or devices.
Several improvements are known in the prior art. Japanese Patent Application Kokai No. 21417/1983 discloses an epoxy resin composition having blended therein a copolymer of an aromatic polymer and an organopolysiloxane. It is also proposed to use an alkenyl group-containing novolak epoxy resin as the aromatic polymer for such a composition as disclosed in Japanese Patent Application Kokai No. 84147/1987 or European Patent Application No. 218228.
Surface packaging is currently one of the most often used practical techniques for semiconductor device packaging. Such advanced packaging techniques often cause the encapsulating resin to be exposed to elevated temperatures of, say, 215.degree. to 300.degree. C., as by infrared reflow or solder bath. In many cases, semiconductor devices are allowed to stand for some time between resin encapsulation and packaging. During standby time the encapsulating resin tends to absorb moisture from the ambient air. When the encapsulated element is subjected to infrared reflow or immersed in a solder bath for packaging, the molded epoxy resin which has absorbed moisture is suddenly heated to elevated temperatures so that the absorbed water may evaporate into vapor. The thus created vapor pressure causes the encapsulating resin to crack to detract from the outside appearance and sometimes induces defects in the semiconductor element or device.
Therefore, epoxy resin compositions, particularly those intended for use in the encapsulation of semiconductor devices, are required to have such properties that cured moldings are resistant to cracking not only in moisture-free state, but also upon heating after moisture absorption.